Method for hardening steels



Patented Apr. 24, 1945 2,374,634 METHOD FOR. HARDENI'NG STEEL Raymond S.Wile, Flushing, N. Y., asslgnor to Induction Heating Com, New York, N.Y., a corporation of New Yorl-r No Drawing. Application January 5,1944,Serial No. 517,116

This invention relates to methods for hardening alloy steels, and isparticularly adapted among other uses, for hardening articles such as ofhigh speed tool steel where it is desired to heat the steel to hardeningtemperature. by electromagnetic induction and in the presence of the airof the atmosphere.

' The use of induction heating in the hardening of high speed toolsteels has heretofore involved dimculty due to the oxidation of thealloying metals of the steel within the surface thereof during theheating step. While his dlmculty may be avoided by surrounding the steelwith a protective gas such as hydrogen, nitrogen or carbon monoxide,such gases as used for the purpose are dlfllcult to control andsometimes introduce factors of danger as in the case of hydrogen whichmay explode, and the use of which is often prohibitive because of fireregulations. Induction heating processes are also most economical, whenperformed with a large number of articles treated in rapid succession,and the use of such protective gases would ordinarily involve rathercumbersome hood structures, or the like, which tend to interfere withthe safe, rapid operation of the induction-heating machine.

It has also been heretofore suggested in lieu of such protective gases,that the steel be coated with sodium silicate, borax glass, or othersalts which are intended to fuse on the surface, and inthatmannerprotect the tool edges from atmospheric degeneration. However,such coatings have the difliculty that they tend upon heating to flowaway from the sharper edges of the tool steel and gather at the heavierportions which need little or no protection.

For these reasons the use of induction heating equipment forthehardening of high speed tool steels has heretofore been relativelylimited.

The present invention provides a method for satisfactorily eliminatingthe above dificulties.

I have found that a mixture of oil such as ordinary mineral oil andfinely divided graphite, pro-= vides a material which may be used as acoating on the steel pieces during the heat treatment and is highlysuccessful in protecting the alloying gietals in the surface of thesteel against oxide.-

The following is a specific example illustrating the invention in one ofits forms, although it will be understood that the invention is notlimited thereto except to the extent indicated in the appended claim.

A steel such as known 7 in the trade as 18-4-1 (comprising 18% tungste4% molybdenum and 1 Claim. (Ci. Bi -l3) 1% chromium) is coated as bybrushing with a mixture comprising, for example, in the neighborhood of50% ordinary mineral oil and 50% of finely divided graphite ground to100 mesh or finer. If desired, the steel article may be coated all over,or if preferred, as in the case of some pieces having sharpened edges,the coating may be confined to the areas comprising or adjacent to suchedges. The coated article is then placed in inductive relation with thework coil of a suitable electromagnetic induction heating machine.Suitable machines for this purpose are shown, for example, in the patentto Paul E. Dravneelr, No. 2,321,189, granted June 8, 19 3. Sufficientcurrent is applied to the work. coil of the machine to quickly heat thesteel piece to the desired hardening temperature, for example wild-230WF. This may be accomplished, for example, during a period of 6 or 7seconds, or usually within a period-of much less than one minute forefiec tlve surface hardening. This rapid heating causes most of the oilto be at once burned off, yet the finely divided g aphite is leftadhering to the surface and acts to fully protect the alloy metals atthe surfaces, even on. the sharp cutting edges, against oxidation. Afterthe surface has become cooled by air quenching or otherwise, the coatingmay be readily removed as by billing or by the use of a wire scratchbrush. it the product is one initially having a bright and pol ishedsurface, such surface may he retained sub stantially unimpaired afterremoval of the coating. Sand blasting Or other harsh means which mightcause a matted surface, are not required for removal of the coating.

Photomicrographs of tool steel articles hardened as above described,except without the coat- ,ing, show a substantial loss of alloy at thecutting edge through oxidation. Photomicrographs of samples treated inthe same way with the coating, however, show a very marked decrease insuch loss, or a practically unobservable alloy loss. A use test oi suchsamples is most reliable, since the usual tests with Rockwell orBrinnell machines may give a false reading because of penetration of anythin film of oxide which may occur at the cutting edge. Under such usetests, with the uncoated samples there was invariably a loss of cuttingaction due apparently to the turning over of the edges. On the otherhand, use tests of the hardened samples which were coated duringheating, with the oil and graphite mixture, give results fully assatisfactory as could be'obtained by heat treating in the presence of aprotective atmosphere oi non-oxidizing gas.

Equally satisfactory results with the invention have been obtained witha wide variety of steels, for example those running as high as 9.8%molybdenum. In no case has there been found any loss of cutting actiondue to oxidation when the steel has been heat-treated with the abovedescribed protective coating.

Such a coating is particularly advantageous for use with inductionheating, since due to the great rapidity with which the heating may beaccomplished, there is insufilcient time for the graphite particles tocombine with the oxygen of the atmosphere at temperatures such as abovementioned. .Also the heating may be readily accomplished by inductionwithout any danger of scarring, disturbing or brushing off the adheringgraphite particles of the coating-as would be likely to occur in thecase of heating with furnaces by the use of fuel.

The percentages of oil and graphite used for the coating mixtures arenot particularly critical, but it has been found that the abovementioned 50% mixture satisfactorily adheres to the steel parts withoutforming a. paste, which would be more difilcult to apply uniformly.While various oils such as vegetable oil, for example as derived fromcorn, may be used, mineral oil appears to be preferable.

The invention is applicable for the "through hardening" of steels aswell as surface hardening.

The term "alloy steels" as used in the appended claim, is intended toinclude those containing one or more of the following metals:molybdenum, tungsten, vanadium, chromium, manganese and other hardeningmaterials.

While the invention has been described in detail with respect to certainpreferred examples, it will be understood by those skilled in the artafter understanding the invention that various changes and modificationsmay be made without departing from the spirit and scope of theinvention, and it is intended therefore in the appended claim to coverall such changes and modifications.

What is claimed as new and desired to be secured by Letters Patent is:

In the hardening of alloy steels, the method steps which comprisecoating the surface of the steel with an adhering mixture comprisingsub- .stantial amounts of oil and finely divided graphitic material,then rapidly heating to hardening temperature by electromagneticinduction in the presence of air, whereby a resulting coating of thegraphitic particles formed on the steel protects the alloying metals inthe surface thereof against oxidation during such heating.

RAYMOND S. WILE.

